Can we have an open and honest discussion about RPMs?

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
Messages
16,012
Display Name
Display name:
Sixer
So I am watching a Trent Palmer special and he casually throws out that redline on his Rotax is like 5400 RPMs...

Record Scratch... What????

Why is my engine limited to 2700?
I thought RPM limits were to keep prop tips from going super sonic. I read that on POA..

Also if it gets 5000 RPM how come it doesn't fly 400kts.

I am confused about physics and everything I have learned on POA.


So I guess the questions are:
why are the redline limits where they are?
why would doubling the RPMs not add tremendous speed to the plane?
 
Two words: reduction gears

Your engine drives the prop directly from the crankshaft so engine RPM=prop RPM. The Rotax drives the prop via a reduction gear or belt. The prop turns slower than the engine.
 
Why would you want an engine to turn twice as fast for the same RPM?
 
SixPapaCharlie said:
Why would you want an engine to turn twice as fast for the same RPM?
Click to expand...

Engine power is mostly dependent on how much oxygen you can pump through it. More RPM pumps more air, makes more power. The trade off is gears & belts add weight & complexity.
 
Any engine has a limit it should be run at so it won’t damage itself. The redline in airplanes probably is lower than that for airplaney reasons.
 
jsstevens said:
Engine power is mostly dependent on how much oxygen you can pump through it. More RPM pumps more air, makes more power. The trade off is gears & belts add weight & complexity.
Click to expand...

I think I don't understand power.
How does adding power to something that has a limit help?

Meaning if readline is 2700 but I have an engine that can spin 5400 so I gear it down so the prop still limited to 2700.
Seems like you're putting a governer on a hot rod.
 
jsstevens said:
Engine power is mostly dependent on how much oxygen you can pump through it. More RPM pumps more air, makes more power. The trade off is gears & belts add weight & complexity.
Click to expand...


That means better efficiency in terms of power versus weight. Rotax engines are very popular for LSAs.

The engine in my motorcycle makes about 130hp from 600cc’s by having a 16,000 rpm redline.
 
SixPapaCharlie said:
I think I don't understand power.
How does adding power to something that has a limit help?

Meaning if readline is 2700 but I have an engine that can spin 5400 so I gear it down so the prop still limited to 2700.
Seems like you're putting a governer on a hot rod.
Click to expand...

Twice as many power strokes per minute. Each stroke doesn’t have to produce as much force. The parts don’t need to be as beefy
 
SixPapaCharlie said:
I think I don't understand power.
How does adding power to something that has a limit help?

Meaning if readline is 2700 but I have an engine that can spin 5400 so I gear it down so the prop still limited to 2700.
Seems like you're putting a governer on a hot rod.
Click to expand...

All 2700s are not equal.
 
SixPapaCharlie said:
I think I don't understand power.
How does adding power to something that has a limit help?

Meaning if readline is 2700 but I have an engine that can spin 5400 so I gear it down so the prop still limited to 2700.
Seems like you're putting a governer on a hot rod.
Click to expand...

The prop isn’t designed to handle 5400 rpms. Take a turbine. The engine (Np shaft) in our helicopter spins at 32,000 rpm. No way the blades could sustain that speed. They’d first hit critical Mach and then rip away from the hub. In order to allow the engine to produce Hp necessary for flight, it spins at a fast rpm and then is significantly reduced to a more manageable rpm (413) at the transmission.
 
Last edited:
Power = torque * rpm => Faster is, in general, more power that you can deliver to the prop.
To keep the prop speed under control with a fast turning engine you gear it down so prop torque = engine torque* gear ratio - More poop to where it counts.

The limits on an engine are usually a function of piston speeds (or sometimes valve open/close rates) wear in the cylinder bore is pretty much a function of how fast the piston is moving inside it. Typically, the sewing machine / 1930's auto engine derived aircraft engines (Lycoming and Continental respectively) turn at slow crankshaft speeds, but have a long piston stroke where a snowmobile derived engine has a very short piston stroke and runs at higher rpm. The net result is that if you calculate the actual speed at which the piston is moving in a straight line - both end up coming out about the same in terms of feet/minute so both "styles" will last about the same number of hours.

Direct drive is simpler (no gears) but require a metric buttload of cubic inches to get the job done. A geared engine can spin faster and make the same power from a smaller engine - but the weight savings of the smaller engine is partially offset by the weight of the gear reduction.
 
Last edited:
SixPapaCharlie said:
That can't be true...

My prop doesn't know what is making it go 2700 RPM.
Click to expand...
But not all props are equal, and the difference is in the load that it carries.
The question is really about how to efficiently produce that power. Look at the difference in a 200hp tractor engine and a 200 hp car engine. The difference is largely rpms.
 
SixPapaCharlie said:
My prop doesn't know what is making it go 2700 RPM.
Click to expand...
The engine in your weed wacker doesn't have enough poop to turn your prop at 2700 without doing something drastic like making the pitch real flat so that it doesn't move as much air. On the other hand, if you took a R3800 and put your prop on it, you would have to add tons of pitch to hold the engine back which would make the prop move biblical quantities of air which would then peel the paint of your airplane.
 
SixPapaCharlie said:
That can't be true...

My prop doesn't know what is making it go 2700 RPM.
Click to expand...
True...but if you tried to turn the same prop with the same Rotax without a gearbox, the engine wouldn't be turning fast enough to develop full horsepower, and therefore you'd never see 2700 rpm on the engine OR the prop. That engine needs to turn 5000+ rpm to make its rated power.
Since the prop has issues at that speed, they gear the engine output to match what the prop needs.
 
Look up the Cessna 175. It had a Continental O-300. But they called it the GO-300. They let the engine run up to 3200 to get 2400 at the Prop through a 4:3 reduction gear. Redline was probably about 3600, just guessing, that would give you 2700 at the Prop. Upping the engine from 2400 to 3200 allowed it produce 30 more horsepower. The plane got a bad rep in engine reliability. Lot of folk couldn’t get their head wrapped around the engine running that fast and ran them where they were used to in other planes, like in the 2400-2500 range. That was bad for the engine because that was in a range where it vibrated a lot. It was ‘balanced’ to run continuously at the higher RPM. Think of the Lycoming O-360’s. They have that avoid continuous operation at certain RPM’s placard. In the 1900 RPM neighborhood if I recall correctly. That’s because it vibrated there because it was ‘balanced’ to run smoothly at cruise speeds.
 
I thought the long pushrods limited rpms? Corvettes (1987) had a redline around 4000. I know my engine seems to run effortlessly at 2600, but 2700 not so much (seems louder and more vibration).
 
Thinking about rotational mass too, our common o-320 and o-360 engines are 320 and 360 cubic inches respectively and they're 4 cylinders. The same displacement engine in a typical automotive application is going to be a V8... so what does this mean? Those airplane engine pistons are comparatively HUGGGE. From what I understand huge engine internals and high RPMs generally don't work well because of the kinetic energy the piston has to overcome when changing direction on each stroke.

Now one figure I do have for every car engine I've ever seen that I've NEVER seen given for aircraft engines is torque. With that displacement and RPM curve I would assume they should have a great deal of low end torque but I've never seen an actual figure.
 
Turning higher rpm also helps fuel efficiency by increase the mixing of the air inside the cylinder due to velocity. A higher rpm engine can be more power dense because you can optimize airflow. At 2,500 rpm the air isn’t moving fast enough to be influenced much by typical hot rod tuning tricks.
 
Ever driven a Honda S2000 vs a Chevy or Ford with a V8? Power delivery comes at different RPM. To make bigger numbers, a smaller motor needs to spin at higher RPM in order to make that power. The V8s make power and torque at a much lower RPM than the small displacement motor.

The Rotax 912 is no different. It’s a small motor that needs to spin at higher RPM to make the 98-100 horsepower, but uses a reduction gearbox to spin the prop at a lower speed. The benefit of a smaller motor is lighter weight and more efficiency. And engine that runs at near WOT where greater airflow results in greater pumping efficiency. (An engine is basically an air pump.)

Note that the Rotax engine is made by Bombardier, which specializes in motorcycle and snowmobile engines, so they’re particularly well practiced as small displacement, higher output engines.

BTW, ever driven a Mazda with a rotary engine? That Wankel engine design is the epitome of small displacement, light weight engines that need to spin to 9,000 rpm to make power.
 
SixPapaCharlie said:
So I am watching a Trent Palmer special and he casually throws out that redline on his Rotax is like 5400 RPMs...

Record Scratch... What????

Why is my engine limited to 2700?
I thought RPM limits were to keep prop tips from going super sonic. I read that on POA..

Also if it gets 5000 RPM how come it doesn't fly 400kts.

I am confused about physics and everything I have learned on POA.
Click to expand...
Gears, as explained, allow the engine to spin faster to provide more horsepower.

SixPapaCharlie said:
So I guess the questions are:
why are the redline limits where they are?
Click to expand...

To keep the engine from coming from together, generally something bad happens over redline, in the engine for instance the valves may not close fast enough to get out of the way of the piston. Or the stress from a connecting rod changing direction faster than it was designed to do can cause it to fail.

SixPapaCharlie said:
why would doubling the RPMs not add tremendous speed to the plane?
Click to expand...

Has to do with the design of the engine and prop. The prop is designed to spin at its most efficient speed, doubling the RPM would cause it to go supersonic and kill its ability to transfer power to air.





SixPapaCharlie said:
Why would you want an engine to turn twice as fast for the same RPM?
Click to expand...

Horsepower, allows a smaller displacement engine to produce more horsepower.
 
Last edited:
eman1200 said:
eman's contribution:

RPM, not RPMs. revolution per minutes doesn't make sense. "2500 RPM", not "2500 RPMs". it's like salmon..."look at all those salmons".

it's ok, u can tell 'em u learn't it on PoA…….
Click to expand...

Sure it is, 5000 rpms or 2500 rpm
 
eman1200 said:
eman's contribution:

RPM, not RPMs. revolution per minutes doesn't make sense. "2500 RPM", not "2500 RPMs". it's like salmon..."look at all those salmons".

it's ok, u can tell 'em u learn't it on PoA…….
Click to expand...
upload_2018-9-29_12-59-59.png
I need more revolutions per minute, RPMs
 
You don't have a 2700 rpm limit motor that can be spun to 5400. RPM is designed into the engine design. Traditional aircraft engines were designed to be low rpm, high torque by using long strokes on the crank and large diameter pistons. Why? Because this is simple, easy to build, low stress, long lasting engineering; perfect for an aircraft (or tractor or generator or water pump or farm impliment) but the downside is they tend to be bigger and heavier. To design an engine to double that RPM to 5-6 k takes better materials (because there is more stress on the parts), more accurate machining tolerances, shorter strokes and lighter (thus smaller) pistons, and less total rotating mass. Grab a pencil or chop stick, Hold one end in your hand and wave it back and forth like your waving a flag. Do it fast. You can go back and forth about as fast as your arm can. No try that with a baseball bat, but wave it slowly back and forth. No problem. Now try and wave the baseball bat as fast as the pencil Not going to happen and its going to take alot more strength and tire you out faster. Thats why you cant spin a Lycoming to 5400. So instead of having a big cylinder giving a big power pulse 11 times a second (2700 rpm) Rotax has designed a motor that gives 22 small power pulses a second (5400 rpm.) Sure each power pulse is half of the the 100 hp 0-200, but the overall amount of work done is equal in the 100 hp Rotax. Think about it like pushing a brick across a parking lot. If you give it a big nudge with your hand and move it one inch once every 2 seconds it will move 30 inches in a minute. If instead you give it a half inch nudge every 1 second, it will also move 30 inches in one minute. The propeller on you airplane takes a certain amount of power to turn, based on its pitch and diameter. How that power is applied to it the prop doesnt care. Build an engine that only has enough power to turn at the props max rpm, fine. Build an engine that turns much faster, but gear it down with a transmission, so it makes the proper amount of power to turn the prop at 2700, fine. Both ways work. Both ways have pluses and minuses. But the point is both systems have to be designed and built from the start as one or the other. You cant convert a Lycoming to spin 5200 rpm, and if you only spin a Rotax to 2700 it is not going to make its rated power.


SixPapaCharlie said:
I think I don't understand power.
How does adding power to something that has a limit help?

Meaning if readline is 2700 but I have an engine that can spin 5400 so I gear it down so the prop still limited to 2700.
Seems like you're putting a governer on a hot rod.
Click to expand...
teejayevans said:
I thought the long pushrods limited rpms? Corvettes (1987) had a redline around 4000. I know my engine seems to run effortlessly at 2600, but 2700 not so much (seems louder and more vibration).
Click to expand...

Engines
 
Cooter said:
But not all props are equal, and the difference is in the load that it carries.
The question is really about how to efficiently produce that power. Look at the difference in a 200hp tractor engine and a 200 hp car engine. The difference is largely rpms.
Click to expand...
teejayevans said:
I thought the long pushrods limited rpms? Corvettes (1987) had a redline around 4000. I know my engine seems to run effortlessly at 2600, but 2700 not so much (seems louder and more vibration).
Click to expand...
Pushrod engines can easily top 10,000 rpm. Watch NHRA Pro Stock racing.
 
You must log in or register to reply here.
Back
Top